Because if you have a stack of fresh meat, that meat will take longer to freeze because it takes up more space and the freezer will also use more energy to freeze it because it is a large mass at room temperature. (I think, correct me if I'm wrong). If you spread it out, it will freeze quicker which will also inhibit bacteria growth.

(It's midnight here, I really should not be replying when I'm sleepy, but I'll deal with it in the morning.)